The problem really lies in making sure you watch your signs. So the error there should be easy to fix.
However, I'd like to comment on this "it's multiple choice." That has nothing to do with it. You really should write all the steps out properly for two reasons: one, problems won't always be this easy and with this few steps, and not carrying everything down will some day cause you difficulties. If you can shortcut a certain amount of stuff, not a problem (except that you messed it up). But second reason is that it makes it easier for other people to help you, and some day you'll have an instructor who will give partial credit and will want to see your work - I don't think you want to miss out on partial credit if your problem is difficult for the instructor to follow.
i.e. the second line should really have been 3 - 10 + 5, so that you're carrying down the "+ 5" with it. That may not mean anything to you now, and you don't have to listen to me, but wait until you're doing big complicated problems. Better to get in the habit now of carrying everything down and following through.
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